Aquatic breeding
Wild fish are all a little different. Some grow faster, some resist disease better. Breeding uses these differences. In selective breeding the farmer keeps only the best fish as parents. Because many traits are passed on through genes, the babies are better on average. This needs many generations.
- Traits chosen: fast growth, good survival, disease resistance, good shape and flesh, low feed use, tolerance to cold or salt.
- Hybridisation: crossing two species or strains to combine good traits, for example hybrid catfish or hybrid striped bass.
- Avoid inbreeding: mating close relatives makes weak fish. Breeders keep records and use many parents.
- Domestication: after many generations the fish become used to farm life.
How much improves? R = h² × S. S is how much better the chosen parents are than the whole group. h² (heritability) is a number from 0 to 1 that tells how much of a trait is passed on.
Biotechnology
Biotechnology uses living cells and their genes to make useful changes. Some tools used for fish and shellfish:
- Chromosome manipulation: a normal fish has two chromosome sets (diploid, 2n). Shocking an egg with heat, cold or pressure just after fertilisation can keep an extra set and give a triploid (3n). Triploid fish and oysters are sterile, so they do not spend energy on eggs and often grow well, and they cannot breed with wild fish if they escape.
- Sex control: in some species one sex grows faster (female sturgeon give caviar, male tilapia grow bigger). Hormones or special crosses make mostly one sex.
- Cryopreservation: freezing sperm in liquid nitrogen to save good lines and to breed later.
- DNA markers: reading small changes in DNA to find fish that carry good genes, even before they grow.
- Genome editing and transgenic fish: changing a gene directly. These need careful safety and ethics rules and are strictly controlled in many countries.
Try it: breed a faster pond
In the 3D pond, run free play. Try top 10 percent versus top 60 percent. With a small top group the average climbs faster. Why is a very small group risky? (Think: inbreeding.)
Key formulas and definitions
- Response to selection: R = h² × S
- S = mean of chosen parents − mean of whole group
- New generation mean = old mean + R
- Diploid = 2n, triploid = 3n chromosomes
- Key terms: selective breeding, hybrid, inbreeding, heritability, triploid, cryopreservation, genome editing
Worked examples
1. The average length in a pond is 20 cm. The chosen parents average 28 cm. Heritability is 0.5. What is the expected average of the next generation?
S = 28 − 20 = 8 cm. R = 0.5 × 8 = 4 cm. Next average = 20 + 4 = 24 cm.
2. A fish has 2n = 48 chromosomes. How many does a triploid of the same species have?
One set n = 24. Triploid 3n = 3 × 24 = 72 chromosomes.
3. A breeder keeps the top 10 fish out of 100. What percent is chosen, and is it a strong or weak selection?
10 / 100 × 100 = 10 percent. Choosing only the top 10 percent is strong selection, but it can raise inbreeding risk if parents are few.
4. A line grows 12 percent faster each generation compared with the one before. If the average harvest weight is 500 g now, what is it after 2 more generations?
After 1: 500 × 1.12 = 560 g. After 2: 560 × 1.12 = 627.2 g.
5. Why are triploid oysters popular in summer?
They are sterile, so they do not spend energy and body mass on eggs and sperm in summer, and their meat stays plump and good to eat.
Common mistakes
- Thinking selection works in one generation. It is a slow gain that adds up over many generations.
- Thinking triploid fish are made by adding a gene. They have a whole extra chromosome set.
- Mixing up inbreeding and selection. Selecting good parents is fine, but mating close relatives gives weak fish.
- Assuming biotechnology always means gene editing. Freezing sperm and DNA markers are also biotechnology.